In recent years, the accuracy of machining which is possible using precision machine tools has progressed substantially, and as a result there is an urgent requirement for apparatus to measure the shape and roughness of a machined surface to a substantially higher level of accuracy than has been possible in the prior art.
The principal method of precision surface shape measurement which has been used on a practical engineering bases until now is based upon homodyne interferency of light. With this method, two light beams of identical frequency are directed onto the surface under measurement, and interference (i.e. homodyne interference) resulting from this is sensed and measured. Such a method provides a maximum level of measurement accuracy of the order 0.1 .mu.m, which is insufficient for many present-day applications.
A modification of the homodyne interference method has been proposed, whereby data representing the phase relationships and amplitude of interference fringes produced by homodyne interference is processed, to thereby derive optical path differences and hence measure surface height variations to a high degree of precision. However such a method requires complex and hence expensive data-processing circuits, which has prevented its practical implementation.
A further disadvantage of prior art types of surface shape measurement apparatus is that in order to scan light beams over the surface, to derive surface shape information, the light beams are generally held in a fixed orientation while body having the surface to be measured is moved with respect to the light beams. The resultant errors which result, due to this physical movement of the surface, set a limitation to the accuracy which can be obtained by practical types of apparatus.